If you are a Group manager you can add custom branding to the header image. In addition you can also display a short Group description. An example of how this looks can be shown below. The bar which displays the different views will remain translucent and the image will be shown behind it.
Changing the image
Navigate to the Group Profile page and click 'change cover photo' button.
You can select one of the default images or upload one from your computer.
Adding a short description
Navigate to the Group Profile page and click 'Edit' in the 'More options' menu.
Add your description to the description field and save your changes. Your description will only be displayed on the Group header if you have uploaded custom branding.